Further analytic approaches have introduced the torus instability [1]} where the flux rope is modelled as a current ring rather than a line current. Rather than a condition on the decay of the background field with altitude, it is instead proposed that for instability to occur the background field component orthogonal to the torus must decrease sufficiently quickly. It has since been shown that the conditions for such an instability are essentially the same as for instability of a line current, and they are just two special cases of a continuous theory of more general current paths [2]}. Although these works provide useful theoretical background as to the nature of flux rope behaviour, they are difficult to relate to physical ropes as these tend to be significantly more complex and much of the physics is by necessity not taken into account. There are indeed other conflicting explanations for instability, such as that of [3]}, who propose that fast magnetic reconnection can occur below a flux rope even if the overlying magnetic field does not decay with altitude.
